Washington — Top US intelligence officials have been warning for years about the threat of “home-grown violent extremists,” or HVEs in their shorthand lingo.

These are typically young Americans, they say, who may feel like outcasts in their own communities but find comfort and purpose in a greater cause – such as, say, violent jihad – often through the Internet.

Minnesota resident Douglas McAuthur McCain, who was killed while battling rebel forces in a Syrian suburb, posted Facebook entries and tweets making it clear that he found camaraderie and a sense of conviction in the Islamic State, also known as IS or ISIS.

Born in Illinois and raised in Minnesota’s Twin Cities area, Mr. McCain is “absolutely the next iteration of the home-grown terrorist threat,” says Matthew Levitt, director of the Washington Institute’s Stein Program on Counterterrorism and Intelligence.

“These home-grown violent extremists are people who keep me up at night,” Attorney General Eric Holder warned in July. “Trying to monitor them, trying to anticipate what it is they are going to do.”

In the case of McCain and others, they may “go over as disaffected young men and come back having forged networks and training – as well as battlefield experience,” says Michael Singh, former senior director for Middle East affairs at the National Security Council, who is now also at the Washington Institute.

An estimated 300 Americans have gone to fight – or attempted to go and fight – in the region, according US intelligence officials. In July, an American from Florida, Moner Mohammad Abusalha, carried out a suicide attack in Syria on behalf of Jabhat al-Nusra, which the United States has labeled a terrorist organization.

With the war in Syria, “the foreign fighter phenomenon continues to grow,” says Dr. Levitt, who adds that the conflict has drawn far more foreign fighters than Kosovo, Bosnia, Iraq, or Afghanistan.

The process of recruiting foreign fighters – specifically, the ways in which IS reaches out to Americans – is also troubling to US intelligence officials.

“IS is the sexiest jihadi group on the block right now, and also the most prominent on social media,” Levitt says.

This in turn has helped the group reach out to Americans far more effectively. As a result, “Our concern is that the radicalization process has sped up so significantly: The timeline is so much shorter, and it’s much harder to identify them in time.”

Before Syria, potential recruits had to be radicalized to the point where they were willing to carry out violent jihad. “Now, with Syria, all you have to do is convince them to defend innocent Sunni women and children against the butcher [Syrian leader Bashar al] Assad, or to go help create the idyllic Islamic state,” Levitt says.

Recently, Levitt says, he met with European intelligence officials who “just had a bunch of 15-year-olds who went for a little jihadi spring break. The ease of travel is so significant in terms of going to Turkey and then south.”

From there, he adds, “They can get radicalized very quickly.”

More troubling, as they return to America, they may decide to carry out attacks “at the behest of others. Or, scarier still – because they don’t send e-mails, make phone calls, or trigger other electronic tripwires,” Levitt notes, “they decide to do things on their own.”